1. Field of the Invention
The present invention relates to a turbo molecular pump, and in particular to a turbo molecular pump which can release heat generated by an impeller without cooling its base section.
2. Description of the Related Art
In case where aluminum chloride or the like is discharged from the turbo molecular pump, the inside of the turbo molecular pump may exceed the saturation vapor pressure where the solidification or adhesion of the produced material occurs. FIG. 6 shows an entire configuration of the turbo molecular pump. In FIG. 6, an impeller 2 is mounted on a rotational shaft 5 for rotation therewith. The impeller 2 is designed so as to rotate while being floated by a magnetic bearing. An electric component section is provided, which includes radial electromagnets 20, 21, radial position detectors 22, 23 and a motor 30 for rotationally driving the shaft. Especially at an impeller 2 and a screw-threaded spacer 4 near a discharge port 26, the solidification or adhesion of the aforementioned produced material is likely to occur around the electric component section. In order to avoid the solidification or adhesion of the produced material, conventionally, a heater band 8 is wound at the periphery of a base section 10 as shown in FIG. 7. However, the excessive heating by the heater band 8 may cause the plastic deformation of the impeller 2. In view of this, such a method is adopted in that a water-cooling pipe 12 is closely contacted with the base section 10. In addition, FIG. 7(A) is a longitudinally sectional view (partially schematic view) of the turbo molecular pump in which the heater band and the water-cooling pipe are disposed, and FIG. 7(B) is a sectional view taken along line 7B--7B of FIG. 7(A). As to the heater band 8 and the water-cooling pipe 12, the heater band 8 and an electromagnetic valve 14 determining the flow rate in the water-cooling pipe 12 are ON/OFF controlled so that the detected temperature by a temperature sensor 16 incorporated in the base section 10 becomes a temperature set by a temperature control circuit 18. With this control, the turbo molecular pump has been regarded such that it is possible to prevent the solidification or adhesion of the produced material while maintaining its performance.
However, if the discharged gas flow rate is set large, the conventional control of the heater band 8 and the electromagnetic valve 14 in the water-cooling pipe 12 may sometimes lead to the problem that the temperature of the impeller 2 becomes twice or more of the temperature set by the temperature control circuit 18 by increasing the discharged gas flow rate when the temperature sensor 16 senses the temperature set by the temperature control circuit 18. For example, if the temperature of the base section 10 is tried to be maintained in a range of from 60.degree. C. to 80.degree. C. under a condition that the discharged gas flow rate should exceed a permissible value, there may arise a case that the temperature of the impeller 2 exceeds its heat-resisting temperature. If this condition lasts for a long time, the stress resistance of the impeller 2 is lowered and the impeller 2 may break may be broken. Therefore, in order to avoid the abnormally high temperature of the impeller 2, the power of the motor 30 must be lowered, or the discharged gas flow rate must be restricted.
The cooling of the impeller 2 is mainly relied on the following two phenomena: The first one is for a case where the discharged gas flow rate is small, and of the heat transmission due to radiation from the impeller 2 to the base section 10 or the envelope 11. The second one is for a case where the discharged gas flow rate is large, and of the heat transmission through the discharged gas by the heat transfer from the impeller 2 to the base section 10 or the envelope 11. These radiated and transferred heats are cooled as a consequence of cooling the base section 10 or the envelope 11.
However, if the generated heat of the impeller 2 is large but the discharged gas flow rate is small, for example in case of no load under a magnetic field, then sufficient heat transmission by the radiation cannot be expected, and the temperature of the impeller 2 may become high.